(a) Field of the Invention
The present invention concerns a method and a device for controlling the erosion of the electrodes of a plasma torch. More specifically, the invention is concerned with the use of the plasma torch under conditions where the electrodes of the torch have a substantially extended life by increasing the surface swept by the extremity of the arc.
More particularly, the invention relates to the controlled displacement of the foot of the arc on the electrode surface of a plasma torch. The controlled displacement of the foot of the arc is obtained by using a time varying magnetic field configuration.
(b) Description of Prior Art
The driving action of a magnetic field on an electrical arc is well known in the art: the value of the driving force on the arc depends on the values of both the arc current J and the magnetic force B according to the following equation: EQU Force=J.times.B.
Furthermore, the direction of the force is perpendicular to both the directions of the current and of the magnetic field.
It is known that the iron and steel industry requests a large amount of energy, the latter being mainly produced by fuels, and that it uses little electricity. For example, in so called integrated plants, coal is preferably used to supply the larger part of the energy required (chemical and thermal) for the reduction of the iron mineral into a metallic state. Coal is also used as a source of energy to manufacture steel by directly converting the metal in an open hearth or in basic oxygen furnaces and coke combustion oven gas, as well as in the other stages of treatment (gas coke oven and gas blast furnace). On the other hand it is known that the non integrated planes must use other forms of energy for melting and heating.
For all kinds of reasons, it would be of interest to convert as many plants as possible to electricity, if this operation would be profitable. However, at the present rates for electricity, especially if conventional equipment is used, it has not been found practical to carry out the conversion. However, the plasma torch could be interesting and practical, especially for the production of a current of hot air or reducing gas for blast furnaces, reforming of fossil fuel for direct reduction, the replacement of electrodes in arc furnaces, scrap preheating, inert gas melting, pellet firing, ingot heating and ladle preheating.
The use of electrical arcs to give very high temperature gases dates from the start of the present century and the apparatus that is used to produce these temperatures is commonly known as plasma torch. These devices have rapidly gone from simple curiosities which are used in laboratories to specialized equipment for the manufacture of unique objects. Recently, the increasing prices and uncertainties with respect to the availability of light hydrocarbon fuels have led the experts to consider the application of plasma torches to a larger number of industrical processes operating at high temperatures. Two main reasons are at the base of this proposition. First, this device enables to reach much higher temperatures, to give much higher heating efficiency than what can be obtained by simple combustion. Secondly, it has been discovered that in many cases, the yields obtained are higher than when operating in conventional manner.
It is known on the other hand that the plasma torches include electrodes which must eventually be replaced because of the erosion produced at the extremity of the arc. Now, if this resplacement takes place at too close intervals, the use of the plasma torch is not profitable, as it is more often the case.
Several patents relate to magnetic fields interacting with electric arcs; U.S. Pat. No. 3,283,205 describes, for example, an arc plasma device where a high frequency magnetic field is used to force the arc column to oscillate laterally with respect to a stream of fluid thus increasing the energy transfer between the arc and the fluid.
Numerous patents relate to the use of a magnetic field to cause an arc to move continuously either on the end of two coaxial annular electrodes or inside two coaxial cylindrical electrodes.
In some applications, as for example in U.S. Pat. Nos. 3,654,513 and 4,439,657, the amplitude of the magnetic field is constant. As the magnetic coils are usually in close proximity of the arc plasma, intensive cooling of the coils must be provided as related in U.S. Pat. No. 4,242,562.
In other applications the amplitude of the magnetic field is varied in time. This is the case of U.S. Pat. No. 4,278,868 where the magnetic field, causing the arc to traverse the metal surfaces to be heated, can be modified in strength or distribution to vary the traverse rate. In still another case (U.S. Pat. No. 4,194,106) the magnetic field is varied to obtain arc-pulsation effects useful for cutting eroding, welding and depositing materials by means of electric arc devices.